Threaded joints are widely used for connecting steel pipes, such as OCTG and riser pipes for use in industrial oil production equipment.
In the past, standard threaded joints specified by API (American Petroleum Institute) standards have typically been used to connect steel pipes for use exploration and production of oil and gas. However, in recent years, the environments in which exploration and production of crude oil and natural gas take place have been becoming increasingly severe, so special high-performance threaded joints referred to as premium joints are being increasingly used.
A premium joint usually comprises, on each pipe, a tapered thread, a metal-to-metal seal portion, i.e., a sealing surface having the ability to form a seal when in intimate contact with the metal-to-metal seal of the other member of the joint, and a torque shoulder portion, i.e., a shoulder surface which functions as a stopper during make-up of the joint.
In the past, since vertical wells were most common, a threaded joint for OCTG could function adequately as long as it could withstand a tensile load due to the weight of the pipes attached to it and could prevent leakage of a high-pressure fluid passing through its interior. In recent years, however, because wells are becoming deeper, because the number of directional wells and horizontal wells having a curved well bore has been increasing, and because the development of wells in severe environments such as offshore or in higher latitudes has been increasing, a wider variety of properties are required of threaded joints, such as resistance to compression, resistance to bending, sealability against external pressure, and ease of handling in the field.
FIGS. 2(a) and 2(b) are schematic explanatory views of an ordinary coupling-type premium joint for OCTG, which comprises an externally-threaded member 1 (referred to below as a pin member, or simply as a pin) and a corresponding internally threaded member 2 (referred to below as a box member, or simply as a box).
The pin member 1 has, on its outer surface, a male thread 11 and an unthreaded portion 12 called a lip which is located at the end of the pin 1 and adjacent to the male thread 11. The lip 12 has a metal-to-metal seal portion 13 on the outer peripheral surface of the lip and a torque shoulder portion 14 on the end face of the lip.
The corresponding box member 2 has, on its inner surface, a female thread 21, a metal-to-metal seal portion 23, and a torque shoulder portion 24 which are portions capable of mating or contacting with the male thread 11, the metal-to-metal seal portion 13, and the torque shoulder portion 14, respectively, of the pin 1.
FIG. 3 is a schematic diagram illustrating the shape and dimensions of a trapezoidal thread, exemplified by an API buttress thread. Most threads for use in premium joints are trapezoidal threads modeled on this API buttress thread. Many threads almost directly copy the dimensions of an API buttress thread with respect to the aspect ratio of the thread teeth, the flank angle, and other features.
In FIG. 3, if the thread is an API buttress thread having a thread pitch of 5 TPI (5 threads per inch), for example, the thread height 74 is 1.575 mm, the load flank angle 71 is 3 degrees, the stabbing flank angle 72 is 10 degrees, and the axial gap 73 between the stabbing flanks is around 100 μm (i.e., 30 to 180 μm) on average.
An overlap in the radial direction called an interference is provided between the sealing surfaces of the pin and the box. When the joint is made up until the shoulder surfaces of the pin and the box abut each other, the sealing surfaces of the two members are brought into intimate contact with each other over the entire circumference of the joint to form a seal.
The shoulder surfaces function as stoppers during make-up, and they also bear almost all of a compressive load applied to the joint. Therefore, they cannot resist a large compressive load unless the wall thickness of the shoulder surfaces is large (or unless the stiffness of the shoulders is high).
When external pressure is applied to a conventional premium joint like that described above, the applied external pressure penetrates through gaps between the threads to a portion 31 shown in FIG. 2 just before the seal portions.
A lip is much thinner in wall thickness than a pipe body, so it can undergo a decrease in radius due to the penetrating external pressure. As the external pressure is increased, a gap forms between the sealing surfaces, resulting in leakage, i.e., a situation in which external fluid enters the inside of a pipe.
If a compressive load is applied to a premium joint in situations such as when OCTG is disposed in a horizontal well or directional well, since most joints have a relatively large gap between the stabbing flanks as is the case with the above-described API buttress thread, the threads have a poor ability to resist compressive loads, so most of a compressive load is borne by the shoulders.
However, the wall thickness (the load bearing area for a compressive load) of a shoulder surface is usually much smaller than that of a pipe body. Therefore, if a compressive load equivalent to 40 to 60% of the yield strength of the pipe body is applied, most premium joints undergo considerable plastic deformation of the torque shoulder portion of the box, resulting in a significant reduction in the sealability of the adjacent sealing surfaces.
The sealability of a joint with respect to external pressure can be improved by increasing the stiffness of the pin to increase its resistance to deformation by radial contraction. For this purpose, a method is often used in which a working process to reduce the diameter of the pipe end called swaging is previously performed to increase the lip wall thickness.
However, if the amount of swaging is too large, in the case of casing, a pipe being inserted into the casing may catch on the swaged portion, and in the case of tubing, the swaged portion may cause turbulence in a fluid such as crude oil flowing inside the tubing and cause erosion. Therefore, the wall thickness of the pin lip wall cannot be increased so much by swaging.
Other conventional techniques for increasing the stiffness of the end of a pin to improve its sealability are described in U.S. Pat. No. 4,624,488 and U.S. Pat. No. 4,795,200. These patents disclose techniques in which sealability is increased by providing a cylindrical portion which does not contact a box at the end of a sealing surface of a pin so as to increase the stiffness with respect to deformation by radial contraction of the periphery of the sealing surface of the pin and to make the sealing surfaces of a joint uniformly contact.
With a pipe joint, even if swaging is performed, it is necessary to provide a tapered thread, a sealing surface, and a shoulder surface within a limited wall thickness. However, in the above-described prior art, the shoulder surface must be disposed in a location other than on the lip because the end of the pin does not abut the box, so the wall thickness of the lip is necessarily reduced.
Thus, there is a limit to the extent to which the stiffness of the lip can be increased so as to resist a decrease in radius caused by external pressure, and the sealability with respect to external pressure cannot be significantly improved. In addition, because the shoulder surface cannot be given a sufficient radial width, a high level of resistance to compression cannot be achieved, and the sealability is poor under a combination of compression and external pressure.
Techniques for giving a thread the capability of bearing a compressive load in order to improve resistance to compression are described in U.S. Pat. No. 5,829,797 and U.S. Pat. No. 5,419,595, for example. U.S. Pat. No. 5,829,797 describes threads in which the load flanks and the stabbing flanks of trapezoidal threads contact each other, and radial gaps are provided at both the thread roots and thread crests. This thread has a very high ability to bear a compressive load because the stabbing flanks are always in contact.
U.S. Pat. No. 5,419,595 describes a thread in which the gap between the stabbing flanks of trapezoidal threads is reduced to 30 μm or less so that the stabbing flanks are brought into contact with each other only when a compressive load is applied. Although the ability of this thread to bear a compressive load is less than that of the thread described in U.S. Pat. No. 5,829,797, it is much higher than that of an ordinary buttress thread.
However, with the thread disclosed in U.S. Pat. No. 5,829,797, if the width of the thread teeth varies, large variations can occur in resistance to compression, anti-galling properties, make-up torque, and other properties. Therefore, it is necessary to make manufacturing tolerances extremely small, and as a result, this thread has the problem that it is unsuitable for mass production and is extremely expensive to manufacture.
U.S. Pat. No. 5,419,595 has a similar problem. Namely, the gap between the stabbing flanks must be set to a value of 0 to 30 μm. In this case, allowable variations in the width of the male thread teeth and the female thread teeth are each only ±7.5 μm, so the thread cutting becomes extremely expensive and unsuitable for mass production.